In electronic applications where tunable, high power, frequency determining elements are needed, the requirements have been filled in a number of ways, each presenting some problems. A series of fixed tuned devices have been used; frequency selection being accomplished by switching from one to another. Where high speed tuning is a requirement, mechanical switches are not fast enough and the switched elements may suffer from radio frequency high voltage break down at higher power levels. Pin diode electronic switches are faster, but introduce losses. Switched resonator schemes become complex and bulky when designed to cover wide tuning ranges. Mechanical tuning of lumped constant elements by operation of an electric motor is too slow in many applications. Since the use of any kind of mechanical tuning technique is generally associated with lumped constant tunable elements, the potential high voltage breakdown characteristics of the lumped constant elements are likely to be a limitation in high power tuned circuits.
Ferromagnetic resonate mode yttrium-iron-garnet (YIG) tuned filters and oscillators have been used for limited power requirements where relatively simple circuits are required; for example, single resonator narrow bandpass filters vs. multi-resonator, arbitrary bandwidth designs. These designs are generally limited in operation to frequencies above one-half gigahertz.
Varactor tuned filters and oscillators have also been built, but they too are limited to low power applications and losses are generally high. The Q and tuning range of varactor tuned devices are also limited at high frequencies.
A system utilizing a variable permeability substrate for tuning a cooperating stripline inductive element has been developed for use in high frequency, high power amplifiers. The variable inductance is associated with a remotely located capacitive element for establishment of a resonant frequency load for the amplifier. This system provides very fast tuning of the amplifier load by adjustment of the static magnetic field in the substrate. The magnetic field is electrically adjusted to provide the desired permeability within the ferrite substrate, thus changing self-inductance of the strip-line to accomplish tuning thereof. This system is not very suitable for use in filter circuits because of the physical difficulties in making electrical connections between the necessary pluralities of inductances and capacitances. The system also suffers from having to withstand relatively high levels of RF voltages in the lumped constant capacitive elements. Further, it is very difficult to accomplish desired RF coupling between the various elements of filters fabricated in this manner so that it becomes impractical to build anything but the most rudimentary kind of filter using this technique and those that are made this way are severely limited in power handling capability.